Hollow core concrete slabs are conventionally produced by an extruding machine with a frame mounted on a pair of rails by wheels on the frame. A pallet extends between the rails. A hopper is carried by the frame and feeds a concrete mix downwardly to a set of augers which are driven to pick up the concrete mix and compress it within a packing chamber to form a slab with parallel longitudinal bores formed by the augers. The compression of the concrete drives the machine forwardly on the rails, with the slab being extruded behind the machine. As the compressed concrete is extruded, a hammering plate of the packing chamber is vibrated using rotary vibrators operating at high speed, conventionally 10,800 RPM, on an axis perpendicular to the direction of extrusion. The vibration initially assists with the concrete flow through the packing chamber and then is used to compact the concrete mix.
In order to provide the required compaction of the concrete at the sides of the slab, it has been conventional to operate the augers adjacent the sides of the slab at a rotational speed greater than that of the remaining augers. It is to be understood that the augers adjacent the sides must feed a greater quantity of concrete into the packing chamber than the interior augers. The high vibration rate has been found necessary to provide a satisfactory product, although the noise generated is a significant problem.
The present invention is concerned with certain improvements in such an apparatus and methods for extruding concrete slabs.